For centuries, external splints of various forms have been used to provide skeletal support to injured or healing limbs. For decades, various forms of external fixation have been used by orthopedic surgeons to support bones that are healing from traumatic fracture, or that are healing from reconstruction surgery intended to correct deformities by repositioning, lengthening or shortening various bone segments. These external fixation devices can use single shafts attached to bone elements by half-pins, or circular arcs or rings which can be attached to bone elements by half-pins or by tensioned wires that pass all the way through the limb (known as the Ilizarov technique).
In general, the goal of these external fixators is to maintain the relative position of two bone segments during healing. The desired relative position may be fixed (as in the case of simple trauma healing) or variable (as in the case of gradual bone lengthening or deformity correction). Also, the desired stiffness of the system may be very high in some cases, such as the initial healing phase of unstable oblique fractures, and may be lower in other cases where some external load sharing by the healing bone is desired. Many types of external fixators have been developed. One of the most sophisticated, adaptable, and easily adjustable of these is the Taylor Spatial Frame, developed by Harold S. Taylor, J. Charles Taylor, et al, and marketed by Smith & Nephew, Inc.
Taylor et al., U.S. Pat. No. 5,702,389 describes a variety of fixator types, and discloses a ring-type external fixator based on a six-degree-of-freedom “Stewart Platform.” In this design, six adjustable-length struts are used to connect a first base member for mounting to a first bone element to a second base member for mounting to a second bone element. Spherical joints which are common to the ends of two different struts are used to pivotably mount the struts to the base members.
Taylor et al., U.S. Pat. No. 6,030,386 retains the same basic structure of six adjustable-length struts connected between two base members, but replaces the spherical end joints with a combination of a rotating joint plus two pivoting joints which all share a common axis so as to allow independent rotation of each strut about its axis, in addition to the required two axis of pivoting required at the mounting ends.
Taylor et al., U.S. Pat. No. 5,891,143 discloses a particular design for a family of base members having different diameters, whereby all of the family members contain a circumferential array of holes with fixed separation, and whose total number is divisible by three. These holes are designed to support mounting blocks holding half-pins, Ilizarov wires, or other hardware which is attached to the bone elements.
The Taylor Spatial Frame provides good range of motion and adjustability, but each of a large number of mechanical joints and threaded parts in each strut adds some inevitable amount of mechanical clearance. The sum of all of these small mechanical tolerances results in a non-negligible amount of mechanical “play” in the system. The possible variations in the precise positioning and the distance between the strut ends create kinematic uncertainty that substantially limits the positional accuracy and precision by which the base members are held.
More specifically, the design of the Taylor Spatial Frame, as shipped commercially and as taught in U.S. Pat. No. 6,030,386, introduces small but necessary mechanical tolerances at several locations on each end of each adjustable length strut, including but not limited to radial and axial tolerances between the shoulder screw and the base member, radial and axial tolerances at each of the two pivot joints at each end, axial thread clearances between the threaded rod and the adjustment nut, and axial clearances between the internal retaining ring and the corresponding retential grooves in both the adjustment nut and non-threaded portion of the strut.
The end result of these cumulative tolerances is mechanical uncertainty (or “play”) of somewhere on the order of 1 mm in any direction, and on the order of 1 degree in rotation about multiple axes. While some units may be substantially more accurate than this, the practical manufacturing tolerances that can be achieved on this many parts, together with the kinematic magnification of errors that can occur in some configurations, means that there may always be some perceptible level of clearance.
This mechanical clearance creates two deficiencies. The first deficiency is the inability of the structure to precisely and rigidly maintain the relative position of the base members. While the overall rigidity of the frame in high for large applied motions, the rigidity for small motions is nearly zero. This can result in unwanted bone motion and the unwanted transmission of external loads to the bones during certain healing phases. Furthermore, this mechanical clearance can result in the generation of acoustic noise in response to applied loads. The acoustic noise can potentially be noticeable enough to attract unwanted attention and/or to disturb the sleep.
An additional deficiency of the current art is the inability to controllably adjust the stiffness (or its inverse, the compliance) of the structure. The compliance of the external fixator determines the degree to which external loads are carried by the fixator frame itself, and the degree to which they are transmitted to and carried by the bone. Generally, it is believed that bone fracture healing and bone regeneration is affected by the level of mechanical rigidity that is provided by fixation devices during the healing process. Furthermore, it is generally believed that the optimal level of fixation rigidity varies during the fracture healing process, with maximum rigidity (i.e., minimum compliance) generally being most appropriate during the initial primary healing or callous formation phases, and with progressively lower rigidity (i.e., higher compliance) being most appropriate during the later callous remodeling phase when load sharing between the bone and the frame is required.
In summary, many current orthopedic fixation devices, such as the Taylor Spatial Frame, have manufacturing tolerances that significantly limit the maximum stiffness that can be achieved for small displacements, and no known available fixation devices provide a means for controllably adjusting the stiffness to a lower level (i.e., increasing the compliance of the fixator) if desired during later healing stages.